Roof drainage systems need to be designed and installed with appropriate overflow provision, as failure to do so can result in serious damage to buildings and their contents. It is necessary to calculate the hydraulic capacity of a box gutter relative to the type of overflow device that is appropriate. The stormwater drainage code AS/NZS 3500.3:2015, entitled Plumbing and Drainage—Part 3: Stormwater Drainage, (herein referred to as “3500.3”) specifies three types of overflow devices for installation with box gutters. These are a rainhead device, a sump with a side overflow device or a sump with a high-capacity overflow device. With each of these devices it is necessary to have regard to the specific, appropriate type of device into which a given box gutter is to discharge when designing a roof drainage system incorporating box guttering.
FIGS. 1 to 3 described later herein illustrate the three types overflow devices for box gutters specified in 3500.3. Of these, the rainhead (FIG. 1) operates in such a manner that an increase in the depth of water flow in the box gutter, operating up to its maximum design hydraulic capacity, is not required when the (normal) downpipe becomes blocked and the stormwater is required to overflow a wall forming a weir at the front of the rainhead and discharge to atmosphere. Conversely, both the sump/side overflow device of FIG. 2 and the sump/high capacity overflow device of FIG. 3 require an increase in the depth of flow in the box gutter when the normal downpipe is blocked, in order for the device to allow overflow to occur up to the maximum design hydraulic capacity of the box gutter.
The present invention relates to an alternative form of overflow device that complies with the objectives underlying 3500.3 and which enables performance at least comparable to devices as specified in 3500.3. The device of the invention can be used as an alternative to the sump/high capacity overflow as currently prescribed by 3500.3 and shown in FIG. 3. However, the device of the invention may be also used in lieu of the sump/side overflow device of FIG. 2, and it can even be adapted for use as an alternative to the rainhead device shown in FIG. 1.